In the roller mill art it has been general practice to construct the mills where the grinding rollers do not move in a circular path but are moved radially toward and away from a rotating bowl having a grinding face presented to the grinding rollers. Another form of roller mill is to present the bull ring in a stationary setting and to rotate the grinding rollers in a circular path relative to the bull ring so that centrifugal force will force the grinding rollers toward the bull ring. The present invention is directed toward the second type of roller mill in which the bull ring is stationary and the means for driving the grinding rollers centrifugally includes means to provide a constant gap between the rollers and the bull ring without in any other way interfering with the free action of the grinding rollers so that the grinding force exerted on the material to be grounded against the stationary bull ring is varied as the square of the speed of rotation of the grinding rollers. In other words, the principle of the present invention is to establish a predetermined gap space between the stationary bull ring and the path traveled by grinding rollers so as to maintain a constant desirable minimum gap space while not offsetting or obstructing the centrifugal force holding the grinding rollers in operative position.
In prior roller grinding mills of the present type it has always been necessary to have sufficient material in the grinding chamber at all times so as to provide a sufficient layer of material to be ground between the grinding rollers and the bull ring, thereby preventing metal to metal contact. There is an important reason why prior mills required such a condition and that is to avoid the resulting vibration and extreme noise that resulted from metal to metal contact, and to avoid the self destructive result.